Arenes

Question 1

What was the original Kekule structure?

Answer 1

Cyclohexane-1,3,5,triene.

3 single bonds and 3 double bonds.

Question 2

What were the three problems with kekule’s model?

Answer 2

Double bonds - the double bonds meant that benzene should react easily with halogens eg turning bromine water colourless. however, it doesn’t do this. Benzene has a very low chemical reactivity.
Bond length- the alternating double and single bonds, kekule’s proposes, should mean alternating bond lengths. However, benzene has 6 symmetrical sides.
Enthalpy changes - kekule’s structure: enthalpy change of -360kJmol-1
However, benzene actually has an enthalpy change of -208kJmol-1. Benzene Is more stable than kekule thought!

Question 3

Describe the delocalised module

Answer 3

• all bonds have the same length and strength
• carbon atoms in ring form 3 sigma bonds; 2 with carbon atoms and 1 with a H atom.
• 1 electron from each carbon atom is delocalised, forming a ring of electron density above and below the plane
• trigonal planar, 120 degrees Celsius

Question 4

Why doesn’t benzene take part in addition reactions?

Answer 4

Addition reactions would disrupt the delocalisation of the ring structure

Question 5

Describe the electrophilic substitution of arenes with conc nitric acid in the presence of conc sulphuric acid.

Answer 5

NITRATION.
overall equation: C6H6 + HNO3 -> C6H5NO2 + H2O
reaction mechanism:
1) formation of NO2+
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
2) NO2 electrophile then reacts with benzene
3) H+ reforms H2SO4 by combining with HSO4- from step 1. Therefor H2SO4 is the catalyst.

Question 6

Describe the electrophilic substitution of arenes with a halogen in the presence of a halogen carrier.

Answer 6

HALOGENATION
Overall equation: C6H6 + Br2 -> C6H5Br + HBr
Reaction mechanism:
1) formation of Br+
Br2 + FeBr3 -> Br+ + FeBr4-
2) Br electrophile then reacts with benzene
3) H+ reacts with FeBr4- from the first step to reform FeBr3

Question 7

Does benzene react readily with Br2? Explain why.

Answer 7

• Benzene is unable to react with Br2 without the help of a halogen carrier.
• this is because benzene has delocalised electrons spread over all 6 carbon atoms. This means there Is a much lower pi-electron density than alkenes. In alkenes, the pi-electron density is concentrated above and below the 2 carbon atoms.
• the halogen carrier generates the more powerful electrophile Br+

Question 7

What is a salt?

Answer 7

When H+ in an acid has been replaced by a positive ion.

Question 8

Describe the reactions of phenol with sodium hydroxide.

Answer 8

Phenol is neutralised by sodium hydroxide to form a salt, sodium phenoxide and water.
C6H5OH + NaOH -> C6H5O-Na+ + H2O

Question 9

Describe the reactions of phenol with sodium

Answer 9

Sodium is added to phenol, metal effervesces -> hydrogen gas is produced, sodium phenoxide is the salt.

Question 10

Describe the reactions of phenol with bromine.

Answer 10

• Room temperature without need for a halogen carrier catalyst.
• When Br2 is added to phenol, the colour changes from Orange to colourless.
• a white precipitate of 2,4,6-tribromophenol is formed.

Question 11

Explain the relative ease of bromination of phenol compared with benzene.

Answer 11

• lone pair of electrons on oxygen p orbital of the phenol interacts with pi electrons of ring.
• this increases electron density, activating ring.
• this increased electron density polarises Br2 molecules so they are attracted more strongly towards the phenol ring.

Question 12

State the uses of phenols

Answer 12

• disinfectants
• antiseptics
• production of plastics
• resin for paint